Arun bought toffees at 6 for a rupee. How many for a rupee he should sell to gain 20%?

You can go through options.
CP = 100 / 6 = 16.66 paisa
SP = 100 / 5 = 20 paisa
Profit (%) = [( 20 - 16.66 ) / 16.66 ] X 100
= [ 3.33 /16.66 ] X 100
= ( 1 / 5 ) X 100 = 20%
Hence, option (c) is correct.
Alternatively:
CP = 100 / 6
SP = ( 100 / 6 ) X 1.2 = 100 / 5
Hence, he should sell 5 toffees for Re. 1 (100 paisa)